In a process of manufacturing a semiconductor device, a new technology is introduced in which a critical dimension (hereinafter, referred to as CD) of a pattern is equal to or less than 30 nm, and the pattern is formed at a smaller pitch. As a typical example, there is a process called a self-aligned double patterning (SADP). In this scheme, a film is formed over all after a resist pattern is formed by a conventional lithography, and subjected to etching to form a pattern in a side wall of the resist pattern. Therefore, it is possible to form a dense pattern of which the pitch corresponds to the half pitch of the pattern created by the lithography in the beginning. Furthermore, in recent years, there is developed a process called a self-aligned quadruple patterning (SAQP) in which film deposition is performed on the pattern formed by the SADP two times to divide the pitch by etching (that is, the SADP is performed twice). An example of such a scheme is illustrated in FIG. 1. FIG. 1 is a cross-sectional view schematically illustrating the pattern for describing a process of forming fine lines and spaces using the SAQP. FIGS. 1(a) to 1(f) illustrate a process of forming a line pattern by a first SADP, and FIGS. 1(g) to 1(i) illustrate a process of forming the line pattern by a second SADP. First, a resist layer 121 patterned in a line shape is formed on a silicon oxide film 126, a silicon nitride film 125, a silicon oxide film 124, a carbon film 123, and an antireflection film 122 which are sequentially stacked from the bottom side (FIG. 1(a)). Next, a silicon oxide film 131 is formed to cover the resist layer 121 (FIG. 1(b)). Further, while the resist layer 121 is illustrated by one in the drawing, a number of resist layers are formed in practice. Next, the silicon oxide film 131 is subjected to an anisotropic etching to form the silicon oxide film 131 of a line shape on both sides of the resist layer 121 (FIG. 1(c)). Next, the resist layer 121 is eliminated by ashing (FIG. 1(d)). Next, a stacked film of the antireflection film 122 and the carbon film 123 is subjected to the anisotropic etching using the silicon oxide film 131 of a line shape as a mask so as to form the stacked film of a line shape (FIG. 1(e)). Next, the antireflection film 122 is eliminated (FIG. 1(f)). Through this process, it is possible to form a dense pattern of which the pitch corresponds to the half pitch of the pattern formed by the lithography in the beginning.
Subsequently, the silicon oxide film 141 is subjected to an anisotropic dry etching after covering the carbon film 123 of a line shape to form a silicon oxide film 141, and thus the silicon oxide film 141 of a line shape is formed on both sides of the carbon film 123 (FIG. 1(g)). Next, the carbon film 123 is eliminated (FIG. 1(h)). Next, the anisotropic etching is performed using the silicon oxide film 141 as a mask to form a stacked film of the silicon oxide film 124 and the silicon nitride film 125 of a line shape (FIG. 1(i)). Therefore, it is possible to divide the pitch further more with respect to the pattern formed by the SADP. Further, the above materials are given as an example, and not limited thereto.
FIG. 1(i) shows a final pattern shape, but there is a possibility to inspect the pattern after the etching mask material left on the top of the line is further eliminated. Herein, the symbols 1, 2, and 3 indicate space portions in the final shape. In addition, the symbols 101 to 108 indicate the edges of interest in the line pattern.
In the SAQP, one line formed by the conventional lithography in the beginning becomes four lines in the end. Therefore, the nature (CD or roughness shape of the edge) of the line pattern and the space pattern arranged in parallel varies in a cycle of four lines and spaces.
Further, a space (mandrel: a space portion indicated by symbol 1 in FIG. 1) of which the center corresponds to the center of the resist line pattern formed by the first lithography in a space group of the final line and space shape formed by the SAQP will be called an initial core in the following. Further, a final space portion obtained from a film deposited on the outside of the side wall of the first resist pattern is defined as a second core (a space portion indicated by symbol 2 in FIG. 1), and the other space is defined as a gap (a space portion indicated by symbol 3 in FIG. 1).
In a dense line pattern created by the SAQP, there is a possibility of a systematic (that is, random) dimension variation which has not been found in other methods. Typical structures are illustrated in FIGS. 2 and 3. FIG. 2 shows a case where the CD of the finally formed line pattern is not even. A situation that the lines are formed thick and fine as illustrated in FIG. 2 is considered as a case where anisotropic film deposition occurs at the time of manufacturing. FIG. 3 shows a case where a space CD is not even. Various factors are considered in the unevenness of the space CD, which will be described below. While wafer managers generally pay attention to a dimension of the line pattern, the position of the line becomes deviated even though the dimension of the line pattern is formed correctly.
A critical dimension scanning electron microscope (hereinafter, referred to as CD-SEM) having a length measuring function has been generally used in inspection of fine patterns without limited to the SAQP. The reason is that the CD-SEM has high magnification and high measurement reproducibility. Furthermore, in recent years, an index for evaluating an output result of the pattern shown in a captured image is proposed not only a simple measurement of the CD or the pitch, and the function of the CD-SEM is increased. When using such a function, it has been indicated that an amount of exposure is insufficient since the CD of the pattern created by the lithography is large. It has been indicated a possibility that a focus setting of an exposure device is not normal since line-edge roughness (hereinafter, referred to as LER) of a line edge is large. Alternatively, dimensions of a plurality of patterns in an image have been calculated one by one. In addition, the accuracy has been improved, and a sufficient sensitivity has been achieved even with respect to the CD value equal to or less than 20 nm. Therefore, the CD-SEM has been an important tool for estimating a problem of a pattern forming process in a mass-production field.
However, the inspection using the CD-SEM becomes important and difficult more than ever in the SAQP process. The reason for importance is that there are a plurality of processes to be passed through such as a normal lithography (that is, exposure and development), twice-film formation, twice-etching, and twice-core pattern elimination until that a final pattern is formed. In a case where the CD is deviated from a target value in a simple lithography so far, a condition of the exposure process is inspected. In addition, exposure, development, film formation, etching, and eliminating the core pattern are performed one time even in the SADP. In the SAQP, the number of processes is increased, and thus a dimension abnormality easily occurs. In addition, the reason for difficulty in the specification is that the number of processes until a time when the final pattern is obtained is increased and thus there occurs a dimension abnormality to which problems of a plurality of processes are related. In the dimension inspection, there is a need to obtain a clue for specifying a process causing an abnormality not only simply inspecting the dimension. There is required an index or an algorithm for indicating a problem from the resultant pattern data to which the plurality of process is related.
In addition, an overlay deviation is also a problem besides the detection of the dimension abnormality in the SAQP process and the estimation of a problematic process. As illustrated in FIG. 3, the deviation of the space CD means that the following problems occur. In a case where the pattern is positioned as designed when the resist pattern is formed at the first time (that is, the pattern is formed at an appropriate position with respect to the pattern of the lower layer), it is determined that an amount of overlay between the layers is sufficiently small. Further, it is expected that the element performance is degraded by the overlay deviation. However, the position of the line pattern is deviated as a result of the SAQP. Therefore, a positional deviation between each pattern and the pattern of the lower layer or the upper layer becomes large. The overlay deviation is not possible to be recognized when the pattern is exposed. Therefore, there is a possibility to cause a defect of an unexpected element. However, a method of detecting such a defect has not been found yet.
The above-described difficulties are two necessary techniques of establishing an important dimension inspection method of an SAQP pattern. As a first technique, there is a technique of distinguishing the line pattern and the space pattern formed by different processes which look like similar but different. In this technique, a position of a space pattern where the first resist pattern (that is, the initial core) has been occupied among four categories of space patterns which are looked similar is specified. As a second technique, after the initial core is specified, an index corresponding to an abnormality in each process such as a first lithography and two times of film formations is calculated so as to calculate an actual overlay deviation amount.
As the simplest method of realizing the first technique, the ends of a line pattern group are set to be arranged in one image by taking a capturing range wide. However, in this method, an area which is not contained in one capture range of the image is not possible to be inspected. An important portion in creating a typical device is the vicinity of the center of the pattern group, and the inspection on an area away from the ends of the line pattern group is essential. A method of widening the size of the image may be considered. However, there is a need to improve hardware in order to realize the method. In addition, it takes a long time to capture a wide range with accuracy. Therefore, there are also demerits such that a throughput is reduced, and a large storage capacity is required for storing the image. Therefore, there is required a method of specifying a core position from a CD-SEM image where the ends of the line pattern group are not contained.
In the case of the SADP pattern, the method disclosed in PTL 1 is proposed. In the case of the SAQP, there are three categories of space patterns such as the initial core, the second core, and the gap. In the case of SADP, only the core and the gap are included. In PTL 1, edges formed by depositing a film on the outside of the core (that is, edges interposing the gap) are formed using a phenomenon that roughness becomes small compared to the edges on the right and left sides of the core area. The roughness of the edge (that is, a line edge roughness) is calculated for each edge, the edges on the right and left sides of the space are paired up, an average LER of the edge belonging to the set is calculated, and the set of edges having a large LER value is determined as belonging to the core.